Do a search for Dixie Tri-Ball...
Never mind, here it is:
http://castboolits.gunloads.com/show...light=tri-ball
Your payload will be less than that but surprisingly not that much! Two 0.662" RB's weigh 842 grs. and three 0.600" RB's weigh 939 grs. if balls are wheelweight, and according to James Gates they should be hard so best water dropped or oven heat treated.
Also, I have a load from the internet (so we know it's safe!) for two 0.690" RB's. I have not tried it but it appears to come from a reliable source on shotgunworld.com and cross referencing to published load data for the payload shows it is reasonable. If you are interested I'll PM you load details.
I was thinking along the same lines but for me 0.600" or 0.610" was what I was looking at. Haven't got there yet. I'll be following this thread to see how you do!
You might do a search for Tri-Ball anyway to check on successes and failures and accuracy. I've wondered if a 2 ball load would pattern better, worse or similar to a Tri-Ball load.
Also of possible interest:
http://www.siarm.com/index.php?cPath=2_20_164_60
scroll to the bottom for 2 ball wads. I'm sure they listed ball diameter somewhere but I can't find it now.
Steel shot wads should work though but i'd put some cushion material between balls.
Longbow